a) Field of the Invention
The invention relates to a universal trailer lock and, more particularly, a universal trailer lock for tightly locking on a trailer coupler, which is applied to trailer couplers with different specifications for effectively prevent trailers from being stolen.                b) Description of Prior Art        
For preventing from being stolen, a trailer lock 2 is usually used to lock on a coupler in front of the trailer. Since couplers may be roughly divided into three specifications: large size, medium size and small size, a general trailer lock applied for a medium-sized or small-sized coupler 1 could not be provided for large-sized coupler 1 to lock, otherwise, a trailer lock applied for a large-sized coupler 1 could not be provided for a medium-sized or small-sized coupler to lock. The following problem may be caused in this condition:
1. For the market demand for the use, manufacturers may produce trailer lock 2 with at least two different specifications, thus, preparation costs and inventory space may significantly increase.
2. For dealers, there are many specifications of trailer locks made by various manufacturers, however, other related goods may also be displayed in limited space, the products with various specifications may only be occupied display space of other products, and stored space may also increase, so that purchase intention of dealers may be affected.
3. For users, while changing to another trailer, if specification of the couplers 1 are different, the trailer lock 2 should be changed, it is not eligible for benefits.